Nowadays, microelectronic products generally can be categorized into two groups, namely logic devices and memory devices. The memory devices are included in almost all of modern electronic products, and thus play a very important role in the microelectronics field. The memory devices generally can be categorized into volatile memories and non-volatile memories. The non-volatile memories are mainly characterized by that they can hold information stored for a long time even if powered down. The non-volatile memories have properties similar to those of Read-Only-Memories (ROMs), and also have very high access speeds. Furthermore, the non-volatile memories are easy to be erased and rewritten and have low power consumption.
Due to requirements of applications such as multimedia and mobile communications to large-capacity and low-power-consumption storage, the non-volatile memories, especially Flash memories, are sharing an increasing portion of the semiconductor device market, and thus are becoming a very important type of memories.
A conventional Flash memory is a silicon based non-volatile memory having a polysilicon-thin-film-floating-gate structure. Typically, the memory has an oxide layer serving as a tunneling dielectric layer, a defect on which will cause a fatal discharging channel. In contrast, a charge-trapping type memory utilizes a trapping layer having a localized charge storage characteristic to implement discrete charge storage. In the charge-trapping type memory, defects on a tunneling dielectric layer will only cause local charge leakage instead of fatal discharging channels, so that charges are more stably held. What is more important is that the localized charge storage characteristic can create a plurality of relatively independent physical storage spots in a single device, so as to implement multi-bit storage and thus improve the storage density. The conventional Flash memory uses a planar structure. Contrary, a memory with a vertical structure can make an effective use of side surfaces, resulting in vertical channels and thus increased channel areas, so as to achieve better device performances.
With the rapid development of the microelectronics technology, semiconductor devices continue scaling down. In addition to continuously increased requirements on programming, erasing, and holding performances of the non-volatile memories, high storage densities, which enable a lower cost, have also become a focus. The three-dimensional integration technology is one of several effective methods for improving the storage density. US patent U.S. Pat. No. 5,825,296 describes a three-dimensional read-only-memory. Patent Application US 2008/0,023,747 discloses a semiconductor memory array based on a multi-stack structure and a method for manufacturing the same. However, the memories described in those two documents have the storage densities still lower than what is required in practice. Therefore, how to improve the storage density is still an important topic in the non-volatile memory research field.